U.S. patent number 5,807,428 [Application Number 08/862,115] was granted by the patent office on 1998-09-15 for slurry coating system.
This patent grant is currently assigned to United Technologies Corporation. Invention is credited to Krishnangshu Bose, David W. LaFlamme, Lester J. Magyar, Terry T. Perry.
United States Patent |
5,807,428 |
Bose , et al. |
September 15, 1998 |
Slurry coating system
Abstract
The present invention relates to a composition for coating
internal surfaces of an airfoil, particular high pressure
temperature airfoils such as vanes and blades. The composition
includes a dry composition consisting essentially of from about 2.5
wt % to about 7.0 wt % aluminum fluoride, from about 5.0 wt % to
about 20 wt % of a chromium-aluminum powder, and from about 75 wt %
to about 92.5 wt % Al.sub.2 O.sub.3. The dry composition is mixed
with water and a cellulose compound to form a slurry which is
thereafter injected into the airfoil. The coating compositions of
the present invention are valuable in that they are capable of
forming an oxidation and corrosion resistant coating on the
internal surfaces of the airfoil simultaneously with the formation
of an exterior corrosion and oxidation resistant coating. A process
for simultaneously forming corrosion and oxidation resistant
coatings on the interior and exterior surfaces of an airfoil is
described herein.
Inventors: |
Bose; Krishnangshu (Manchester,
CT), Perry; Terry T. (Bar Mills, ME), LaFlamme; David
W. (Colchester, CT), Magyar; Lester J. (Wallingford,
CT) |
Assignee: |
United Technologies Corporation
(Hartford, CT)
|
Family
ID: |
25337705 |
Appl.
No.: |
08/862,115 |
Filed: |
May 22, 1997 |
Current U.S.
Class: |
106/14.44;
106/14.05; 106/14.11; 106/14.21 |
Current CPC
Class: |
C09D
5/08 (20130101); C23C 10/28 (20130101); C23C
10/18 (20130101); C09D 5/10 (20130101) |
Current International
Class: |
C23C
10/18 (20060101); C23C 10/00 (20060101); C09D
5/08 (20060101); C09D 5/10 (20060101); C23C
10/28 (20060101); C09D 005/08 () |
Field of
Search: |
;106/14.05,14.11,14.21,14.44 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4132816 |
January 1979 |
Benden et al. |
5217757 |
June 1993 |
Milaniak et al. |
5366765 |
November 1994 |
Milaniak et al. |
|
Foreign Patent Documents
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|
|
|
|
|
|
2007494 |
|
Feb 1994 |
|
RU |
|
390196 |
|
Nov 1973 |
|
SU |
|
406969 |
|
Jun 1974 |
|
SU |
|
443941 |
|
Jul 1975 |
|
SU |
|
1145054 |
|
Mar 1985 |
|
SU |
|
1168626 |
|
Jul 1985 |
|
SU |
|
1523594 |
|
Nov 1989 |
|
SU |
|
1539235 |
|
Jan 1990 |
|
SU |
|
Primary Examiner: Cameron; Erma
Attorney, Agent or Firm: Bachman & LaPointe, P.C.
Claims
What is claimed is:
1. A composition for providing an oxidation and corrosion resistant
coating on internal surfaces of an airfoil, said composition
including a dry composition consisting essentially of from about
2.5 wt % to about 7.0 wt % aluminum fluoride, from about 5.0 wt %
to about 20 wt. % of a chromium-aluminum powder, and from about 75
wt % to about 92.5 wt % Al.sub.2 O.sub.3.
2. The composition of claim 1 wherein said chromium-aluminum powder
comprises a chromium-45 aluminum powder.
3. The composition of claim 1 further comprising a cellulose
compound and water added to said dry composition so as to form a
slurry composition to be injected into said airfoil.
4. The composition of claim 1 wherein said dry composition consists
essentially of from about 75 wt % to about 80 wt % of Al.sub.2
O.sub.3, from about 2.5 wt % to about 5.0 wt % aluminum fluoride,
and from about 15 wt % to about 20 wt % chromium-aluminum
powder.
5. The composition of claim 4 wherein said dry composition includes
from about 75 wt % to about 77.5 wt % Al.sub.2 O.sub.3.
6. The coating composition of claim 3 wherein said slurry
composition is used to coat external surfaces of said airfoil.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a process for coating an airfoil
to protect it against oxidation and corrosion during operation and
to a particular coating composition used to form an oxidation and
corrosion resistant coating on internal surfaces of said
airfoil.
Aluminide coatings are applied on nickel-based superalloy turbine
airfoils to protect the airfoils against oxidation and corrosion
during operation in the turbine section of engines. These coatings
are formed by the deposition of aluminum onto the surfaces of the
airfoils. A reaction occurs between the nickel in the airfoil
material and the deposited aluminum to form nickel-based
aluminides. At high temperatures, in the presence of air, the
aluminum in the nickel aluminide coating forms a thin, adherent
aluminum oxide at the surface of the coating. This aluminum oxide
provides a barrier against further oxidation and corrosion of the
turbine airfoil. The external surfaces of turbine airfoils in most
engines are coated with aluminides. The performance requirements of
the engines determine whether aluminide coatings also are required
on the internal surfaces of the airfoils.
Currently, there are two processes used to internally coat
airfoils. One process employs a slurry technique and is used for
airfoils that operate in the low pressure turbine section of an
engine. The second process uses chemical vapor deposition to coat
the internal surfaces of the airfoils for the high pressure turbine
section of an engine. Different coating processes are employed for
the two different types of airfoils because the high pressure
turbine section of an engine operates at a higher temperature and
pressure than the low pressure turbine section of an engine. As a
result, the coating applied to high pressure turbine airfoils must
have higher temperature capacity and must be more robust than those
applied to low pressure turbine airfoils.
U.S. Pat. No. 5,366,765 to Milaniak et al. describes a slurry
technique for coating internal passages in low pressure turbine
airfoils. The slurry described in this patent cannot be used to
coat the internal passages of high pressure turbine airfoils for
the following reasons:
(1) the slurry produces a coating that is too brittle;
(2) the coating is too thick to apply to the internal cooling
passages of high pressure turbine airfoils; and
(3) it is not compatible with the processes used to coat the
external surfaces of airfoils.
As previously mentioned, chemical vapor deposition processes are
used to coat airfoils used in the high pressure turbine section of
an engine. During the coating process, turbine airfoils 10 are
placed in an upright position within a compartmentalized, large
metal box or coating fixture 12, called a coat boat. The FIGURE
illustrates a typical coat box arrangement. To coat the internal
passages of the airfoils 10, chemicals 14 are placed in a
compartment 16 below the airfoils. The airfoils are mounted on
specialized plumbing tools 18 that allow vapors to flow through the
internal cooling passages of the airfoils. Argon gas is introduced
into the lower compartment 16 via inlet 20 to force the coating
vapors through the internal areas of the airfoils. These vapors
react with the internal surfaces of the airfoil to produce an
aluminide coating. At the same, chemicals 14 in an upper
compartment 22 create vapors which react with the external surfaces
of the airfoil to form an aluminide coating thereon. There are
problems however associated with this process. The problems include
the need to use a forced argon flow and the need to use specialized
plumbing tools to allow the coating vapors to flow through the
internal passages of the airfoil.
Thus, there remains a need for a coating process which eliminates
the problems associated with the chemical vapor deposition
processes currently employed. There is also a need for a coating
process which allows the external and internal surfaces of an
airfoil to be coated during a single cycle.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
composition for forming a corrosion and oxidation resistant coating
on internal surfaces of airfoils.
It is a further object of the present invention to provide a
coating composition as above which is compatible with processes
used to coat external surfaces and therefore allows the internal
surfaces of an airfoil to be coated simultaneously with the
external surfaces of the airfoil.
It is yet a further object of the present invention to provide a
coating process which does not need any forced argon flow and/or
specialized plumbing within the coat box.
The foregoing objects are attained by the coating composition and
the coating process of the present invention.
In accordance with the present invention, a composition for
providing a corrosion and oxidation resistant coating on the
internal surfaces of an airfoil includes a dry composition
consisting essentially of from about 2.5 wt % to about 7.0 wt %
aluminum fluoride, from about 5.0 wt % to about 20 wt % of a
chromium-aluminum powder, and from about 75 wt % to about 92.5 wt %
aluminum fluoride. The dry composition is mixed with water and a
cellulose compound to form a slurry composition which can be
directly applied to the internal surfaces of an airfoil, thereby
eliminating the need for an argon purge and the need for
specialized plumbing.
In accordance with the coating process of the present invention,
the aforementioned slurry is formed by providing the aforementioned
dry chemical composition and mixing the dry chemical composition
with the cellulose compound and water. Thereafter, the slurry
composition is placed into direct contact with the internal
surfaces of the airfoil. After the airfoil with the slurry
composition therein has been baked so as to remove the water and to
harden the solids in the slurry composition, the airfoils are
placed within a coating fixture. Also placed in the coating fixture
is a chemical composition for coating the exterior surfaces of the
airfoil. Thereafter heat is applied for a time sufficient to form a
protective coating on the exterior surfaces of the airfoil and
simultaneously form a protective coating on the internal
surfaces.
Further details of the composition and the process of the present
invention, as well as further objects and advantages, are set forth
in the following detailed description and the accompanying
drawings, wherein like reference numerals depict like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The FIGURE illustrates a prior art coating fixture for coating
internal and external surfaces of an airfoil.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
A first aspect of the present invention is the use of a slurry
composition to coat the interior surfaces of airfoils such as high
pressure turbine blades and vanes. As previously discussed, the
primary purpose of the coating formed from the slurry of the
present invention is to increase the oxidation and corrosion
resistance of nickel alloy turbine airfoils.
Compositions in accordance with the present invention for coating
the internal surfaces of an airfoil include a dry composition
consisting essentially of from about 2.5 wt % to about 7.0 wt %
aluminum fluoride, from about 5.0 wt % to about 20 wt % of a
chromium-aluminum powder, and from about 75 wt % to about 92.5 wt %
Al.sub.2 O.sub.3. The chromium-aluminum powder in the dry
composition preferably comprises a chromium-45 aluminum powder. The
aluminum fluoride and the chromium-aluminum powder form the active
elements in the coating compositions of the present invention. In a
preferred embodiment, the dry composition consists essentially of
from about 2.5 wt % to about 5.0 wt % aluminum fluoride, from about
15 wt % to about 20 wt % of the chromium-45 aluminum powder, and
from about 75 wt % to about 80 wt % Al.sub.2 O.sub.3. In a most
preferred embodiment, the Al.sub.2 O.sub.3 constituent is in a
range of from about 75 wt % to about 77.5 wt %.
In order to be applied to the internal surfaces of the airfoil, the
dry composition must be converted into a slurry composition. This
is accomplished by mixing the dry composition with non-active
ingredients including water and a cellulose compound known as
METHOCEL.RTM. Suitable slurry compositions can be formed by adding
water in a range of from about 1900 to about 2700 cc and by adding
the cellulose compound in a range of from about 60 to about 100
grams. The water may be heated to aid in dissolving the cellulose
compound. Any commercially available cellulose compound is usable.
A preferred compound is METHOCEL.RTM. brand cellulose compound
which is distributed by The Dow Chemical Company, Midland, Mich. A
suitable coating slurry can be manufactured by adding 2300 cc of
water and 80 grams of Methocel to the dry compositions of the
present invention.
After the slurry composition has been formed, it is placed into
direct contact with the internal surfaces of the airfoil. This is
done by injecting the slurry into the interior of the turbine
airfoils. Typically, the airfoils have internal passageways and the
slurry is injected into the passageways. Any suitable means known
in the art may be used to inject the slurry into the internal
passages of or otherwise place it into contact with the internal
surfaces of the airfoil. For example, the slurry may be placed into
contact with the internal surfaces of the airfoil using the
technique shown in U.S. Pat. No. 5,366,765 to Milaniak et al.,
which is hereby incorporated by reference herein. Once the slurry
composition has been injected into the interior of the airfoil, the
airfoil is baked to remove water and harden the slurry solid.
Typically, the baking operation consists of subjecting the airfoils
with the injected slurry therein to a temperature in the range of
from about 1025.degree. F. to about 1075.degree. F. for a time in
the range of from about 30 minutes to about 2 hours. Any suitable
means known in the art may be used to bake the airfoils.
The coating compositions of the present invention are compatible
with the CVD processes used to coat the external surfaces of
airfoils. Thus, it is possible using the coating composition of the
present invention to simultaneously form oxidation and corrosion
resistant coatings on both the exterior and the interior surfaces
of the airfoil. To do this, a number of airfoils containing the
baked slurry compositions are placed in an upright manner in a
metal coating fixture similar to that shown in the FIGURE. The
chemical composition for coating the external surfaces of the
airfoil is also placed in the fixture. One suitable exterior
surface coating composition which can be utilized consists
essentially of 15.4 wt % aluminum fluoride and 84.6 wt % of
chromium-45 aluminum. When heated to temperatures above
1975.degree. F., the exterior surface chemical composition
vaporizes and deposits aluminum onto the exterior surfaces of the
airfoils. The internal surfaces of the airfoil are simultaneously
coated by direct diffusion of aluminum from the hardened slurry
composition. A typical thermal cycle applied during this
simultaneous coating operation comprises an initial coat cycle
during which a temperature in the range of 1950.degree. F. to about
2050.degree. F. is applied for a time in the range of 4 to 10 hours
and thereafter a diffusion heat cycle is performed by applying heat
at a temperature in the range from about 1950.degree. F. to about
2050.degree. F. for a time in the range of 4 to 7 hours. As final
steps in the process of the present invention, the airfoils are
heat tint treated at a temperature of about 1075.degree. F. for
about 1 hour to determine coat quality and thereafter precipitation
heat treated at a temperature of about 1300.degree. F. to about
1600.degree. F. for a time period in the range of 12 to 32
hours.
To demonstrate the coating which can be obtained using the present
invention, six slurry compositions were prepared in accordance with
the present invention. The six slurry compositions contained the
dry compositions set forth in Tables 1 and 2. A seventh slurry
composition was prepared using the PWA 273 dry composition set
forth in Tables 1 and 2. After each of the dry compositions was
prepared, 2300 grams of water and 80 grams of Methocel were added
to form a slurry. The various slurries were then injected into the
internal passages of a number of high pressure turbine airfoils.
The coating trial was then completed at a coating temperature of
1975.degree. F. for a coat time of 4 hours. The objects of the test
were to obtain a coating thickness between 1.5 and 2 mils and that
there be no bare spots.
TABLE 1 ______________________________________ Avg. Coat Max. Coat
Min. Coat Dry Thickness Thickness Thickness Slurry Composition
(mils) (mils) (mils) ______________________________________ 1 2.5
wt % AlF.sub.3 0.75 1.7 0 5 wt % Cr-45Al 92.5 wt % Al.sub.2 O.sub.3
2 5 wt % AlF.sub.3 0.57 1.7 0.0003 5 wt % Cr-45Al 90 wt % Al.sub.2
O.sub.3 3 2.5 wt % AlF.sub.3 0.98 1.7 0.0005 15 wt % Cr-45Al 82.5
wt % Al.sub.2 O.sub.3 4 5 wt % AlF.sub.3 1.4 1.7 1.0 15 wt %
Cr-45Al 80 wt % Al.sub.2 O.sub.3 5 5 wt % AlF.sub.3 1.8 2.3 1.4 20
wt % Cr-45Al 75 wt % Al.sub.2 O.sub.3 6 2.5 wt % AlF.sub.3 1.75 2.0
1.5 20 wt % Cr-45Al 77.5 wt % Al.sub.2 O.sub.3 7 5 wt % AlF.sub.3
2.0 3.0 1.2 (PWA 273 30 wt % Cr-45Al composition) 65 wt % Al.sub.2
O.sub.3 ______________________________________
TABLE 2 ______________________________________ Avg. Coat Max. Coat
Min. Coat Dry Thickness Thickness Thickness Slurry Composition
(mils) (mils) (mils) ______________________________________ 1 2.5
wt % AlF.sub.3 0.82 1.7 0 5 wt % Cr-45Al 92.5 wt % Al.sub.2 O.sub.3
2 5 wt % AlF.sub.3 0.87 1.5 0 5 wt % Cr-45Al 90 wt % Al.sub.2
O.sub.3 3 2.5 wt % AlF.sub.3 1.4 2.0 1.0 15 wt % Cr-45Al 82.5 wt %
Al.sub.2 O.sub.3 4 5 wt % AlF.sub.3 1.3 1.5 1.0 15 wt % Cr-45Al 80
wt % Al.sub.2 O.sub.3 5 5 wt % AlF.sub.3 1.8 2.1 1.5 20 wt %
Cr-45Al 75 wt % Al.sub.2 O.sub.3 6 2.5 wt % AlF.sub.3 1.8 2.2 1.5
20 wt % Cr-45Al 77.5 wt % Al.sub.2 O.sub.3 7 5 wt % AlF.sub.3 0.83
2.0 0 (PWA 273 30 wt % Cr-45Al composition) 65 wt % Al.sub.2
O.sub.3 ______________________________________
As can be seen from the data in Tables 1 and 2, the coating
compositions of the present invention met the target goals.
The coating compositions of the present invention yield a coating
which provides oxidation and corrosion resistance under the
conditions at which high pressure turbine airfoils, such as high
pressure turbine blades and vanes, operate. These coatings are
achieved in an economically beneficial fashion without the need for
coating fixtures that have a bottom compartment connected to an
argon flow pipe and without specialized plumbing therein.
As can be seen from the test data reported above, the coating
compositions of the present invention resulted in a uniform coating
within these desired coating parameters needed for repeatable,
robust coating processes.
Although this invention has been shown and described with respect
to detailed embodiments thereof, it will be understood by those
skilled in the art the various changes, omissions, and additions in
form and detail thereof may be made without departing from the
spirit and scope of the claimed invention.
* * * * *